You don't want to use the average lifespan, you want to use the average age of becoming a parent. Note many people become a parent more than once so you want the average parent across all births not the average age of first becoming a parent. This is certainly lower than 35. And for most of human prehistory was probably less than 20. But using 20, the answer is 100 to 200 generations.

Good call; I'd just happened to have been curious about that question the other day, and remembered a couple of the data points from reading up on it.

My education is in the humanities, which, while it sounds like it'd make me an expert at answering this question, actually means some quick division is about all I'm good for!